This present application relates generally to methods, systems, and/or apparatus pertaining to polishing and/or machining metals. More specifically, but not by way of limitation, the present application relates to improved methods, systems, and/or apparatus pertaining to the electrochemical polishing and/or machining of metals, and, particularly, to the electrochemical polishing and/or machining of hard metals, including, for example, those metals used in manufacturing the blades of turbine engines.
In general, there are two primary alternative machining and polishing methods that are most commonly used for hard metals or materials that would otherwise be very difficult to machine with traditional techniques (i.e., those machining methods that rely on mechanical forces, such as turning, milling, grinding, drilling, etc.). These two methods are electrical discharge machining (hereinafter “EDM”) and electrochemical machining (hereinafter “ECM). As one of ordinary skill in the art will appreciate, each of these alternative methods has advantages and disadvantages associated with its usage.
EDM is often called “spark machining” because it removes metal by producing a rapid series of repetitive electrical discharges. These electrical discharges are passed between an electrode and the piece of metal being machined. The small amount of material that is removed from the workpiece is flushed away with a continuously flowing fluid. The repetitive discharges create a set of successively deeper craters in the work piece until the desired shape is produced.
There are two primary EDM methods: ram EDM and wire EDM. The primary difference between the two involves the electrode that is used to perform the machining. In a typical ram EDM application, a graphite electrode is constructed with a specific-shaped and, being connected to a power source and a ram, is slowly fed into the workpiece. The entire machining operation is usually performed while submerged in a fluid bath. The fluid generally serves the following three purposes: 1) flushes material away; 2) serves as a coolant to minimize the heat affected zone (thereby preventing potential damage to the workpiece); and 3) acts as a conductor after breakdown for the current to pass between the electrode and the workpiece. In wire EDM, a very thin wire serves as the electrode. Special brass wires are typically used. The wire is slowly fed through the material and the electrical discharges cut the workpiece. Wire EDM also is usually performed in a bath of water. The wire itself does not touch the metal to be cut; the electrical discharges actually remove small amounts of material and allow the wire to be moved through the workpiece. A computer typically controls the path of the wire.
EDM may be used effectively to machine hard metals or metal alloys, such as titanium, hastelloy, kovar, and inconel, and, moreover, may be used cost-effectively to produce intricate or complex shapes. However, in applications that require a finished product with an exceptionally smooth surface, EDM has a significant disadvantage. In one aspect, this disadvantage includes the formation of a recast layer along the surface of an EDM machined part. A recast layer is a relatively thin surface layer that forms due to the heat of the EDM process. The heat softens areas of the part adjacent to the machined areas, which reharden after the EDM process is complete. The rehardening generally negatively affects the material properties of the metal. One of these negative effects is an increased surface roughness, as typically the rehardening causes the formation of surface defects, burrs, cracks, etc. As one of ordinary skill in the art will appreciate, for many industrial applications, including ones involving hot-path components in turbine engines, surface smoothness may be a prized characteristic. For example, in the case of some turbine engines, achieving a surface finishing of 10-15 RMS may significantly increase the efficiency of the engine, which, of course, is highly desirable in power generating applications. As a result, while EDM is a cost-effective and efficient method for many machining applications, often a second machining or polishing method is necessary to remove the recast layer and smooth the outer surface of the machined part.
ECM also uses electrical energy to remove material from metals. An electrolytic cell is created in an electrolyte medium with two separated electrodes: a tooling piece, which serves as the cathode, and a workpiece, which, being the part being machined by the process, serves as the anode. A high-amperage, low-voltage current is typically used to dissolve and remove material from the workpiece, which, similar to EDM, must be electrically conductive. ECM is essentially a deplating process that utilizes the principles of electrolysis.
During the process, the tooling piece, which, per conventional methods, must be uniquely formed for each different machining application, is positioned very close to the workpiece and a low voltage is applied across the gap (hereinafter “inter-electrode gap”) between the tooling piece and the workpiece. A typical ECM system circulates an electrolyte through the inter-electrode gap such that a high amperage DC current is passed between the two electrodes. Material is removed from the workpiece and the flowing electrolyte solution washes the ions away. These ions form metal hydroxides that, generally, are removed from the electrolyte solution by centrifugal separation. Both the electrolyte and the metal sludge may then be recycled. Unlike traditional cutting methods, workpiece hardness is not a factor, making ECM, like EDM, suitable for difficult-to-machine materials.
There are several advantages associated with ECM. First, the components are not subject to either thermal or mechanical stress during the machining process. As such, unlike EDM, no recast layer is formed. Second, there is no tool wear during the process. The tooling piece, thus, may be used repeatedly without suffering significant wear. Once a specialized tool piece or tool is formed, complex geometrical shapes may be machined repeatedly and accurately by the same tooling piece. Third, ECM may be used to machine or polish surfaces to a very high level of smoothness. In general, surface smoothness of 10-15 RMS or less are achievable.
However, as one of ordinary skill in the art will appreciate, ECM has disadvantages as well. In general, ECM is time-consuming and expensive when compared to other machining methods. This is generally due to the fact that specialized ECM tooling pieces must be constructed for use with each component or part being machined. In addition, as described in more detail below, the complexity of these tooling pieces is generally increased due to the numerous flow channels that are required. Further, a conventional ECM machine is complex and relatively expensive due to the required precise computer controlled movement of the tooling piece relative to the workpiece under high fluid pressure.
Though the invention described herein is not limited to this usage, an example is provided below that focuses on the machining of turbine engine blades. It will be appreciated that this is provided as an example only and that the present invention is not so limited. This example, however, will demonstrate how machining according to the present application may be used to reduce machining costs and increase machining efficiency, particularly for applications similar to the example turbine engine application described.
Turbine engines generally have many stages of rotor and stator blades that may be found in either the compressor, if present, or the turbine section of the engine. Each of these blades has its own set of aerodynamic criteria and, because of this, the blades within each row have their own distinct shape. It will also be appreciate that, as stated, a higher level of surface smoothness generally increases the aerodynamic performance of the blades, which, thereby, improves the overall performance or efficiency of the engine. Given the level of desired smoothness, ECM presents a preferred alternative for machining or polishing the outer surfaces of the blades. Further, for blades that have undergone EDM as part of their fabrication process, ECM provides an attractive alternative for removing the thin outer-layer of recast. This may be done such that, once the recast is removed, a smooth surface remains that performs well in a turbine engine. In this manner, ECM and EDM processes may be used to compliment each other, i.e., EDM may be cost-effectively used for the bulk of the required machining while ECM may be used to produce a finely polished surface of the type that is especially valued in turbine engine applications.
However, because of the many different shapes of blades needed for a turbine engine, conventional ECM becomes a relatively expensive process. In general, the reason for this is that an unique set of ECM tooling pieces is required for each of the many different types of blades that are used within the turbine engine. When the time and cost required for the production of the required tooling pieces is factored into the cost of the ECM machine and the time and cost related to the ECM process itself, ECM often becomes cost-prohibitive for applications of this nature. This is particularly true in industries where parts and components are regularly redesigned or tweaked such that new tooling pieces may be regularly required. As a result, there is a need for improved methods, systems, and/or apparatus relating to ECM processes and ECM machines, and, particularly methods, systems, and/or apparatus that allows ECM machines and processes to be more cost-effective and efficient in terms of initial machine costs, the tooling piece production costs, and/or the labor and time associated with its usage.